Water resistant porous medium



Feb. 13, 1951 L, R, SHARDLOW 2,541,838

WATER RESISTANT POROUS MEDIUM Filed June l2, 1947 f2 f4, ff' f5 IN VEN TOR. [wref/vai ,E5/wea aw 'Parenteel Feb. 13, 1951 STATES PATENT QFFICE.

Y 2,541,838y WATERRESISTANT ronbosMEDIUMv Lawrence Russel Shariilloxm` Nbnthi Arlington..

N. J., assignorto-Rado Corporation of America, acorporationofilielaware;

Appli-Cauca June 12, 19471seriarNt. 7,541,159

5v claims. (C1. 18s-44) Thev present invention relates to filters` of"4 the =type constituted of a porous body and morel especiallyetoltersof this type designed to permitfthe towci" gases-1 therethrough and serving asy a= stop mea-nsfor the flow: of' liquids, and to a novel method of manui-"acture` of 'such' lters.

A porous body is wellA suited as alter, since the minute inter-communicating spaces therein provided serve as a screening medium Thus, if it: isede'sired to` remove solid particles: of predeterminedy dimensions` from aA liquid; thel liquid is caused to flowv orf" pass through a`4 porous body having poresof smaller" dimensions than those of therethrough and as bar to theflow of liquids,

' face tension of' the liquidi with` theA Wallsof the pores which is characteristic of capillary action.

nattemptE to meet this difficulty immediately gives* rise to* others While*- it vis` recognized thatV the capillaryaction of the-liquidbin thepores can be prevented by making the Walls non-Wettable by' the liquid; such forexample asbycoating* the body* andi the2 Walls of.' the:` pores with a non-'wett'ahle medium; several objections to this proceduremake themselves manifest. w

` @ne objection concerns the"lv requirement that coating` adhere to the surfaces over' appre- "ci'allly'y long periods of' time. Most oilsf that are suggejstecifo1-use3in this` connection drylv upv in elative'ly shortrtime endlose-:their characterisetxcfoffreedom fromiwetting: Another` objection resides in: theten-dency of the: oil 'to l ol'osethel pores of-f the-filterbody;v This is" particularly true-where 'coating vthe@vl/Jalis ofthe-pores has advantageslih that its viscosity can be considerably reduced by the application ofi heat' so that it readily venters xthe pores. ThenV o'rrcoolingv itlforins a. relatively hard:coatinglthatpermanently-adheresftothe pore 'surfaees Butthelohjectionf to-theuse cfparafnef tfsathat" i't- IrardensA relatively fasst on coming in contact-'with thelrelatvely coolE wallsioff thev pores, andi this results in' the formation o'f' a relatively thine coating qnitne pore walls; atA least ati loca#- 'ti"o ns:y adjacent@ theJ external surfacesffof the inter.

vliquids at: varying" pressures'.v

- in; particularityv in the appended'.

'liquids ther-'ethrougl'i` results fromvv the fact that the lterf may be' required' to block thel ow of Thus some; uses to` which ai lter'- ofi thissortmay be'. put. involves a1constant contactrwith a: liquid; Othery uses; may sub'j ectthe': lter-to only occasional. contact'szwith liquids-1 A iilter` havingl apore` size for blocking liquids: atA a; maximum: pressure. would. not;` be serviceable with. higher` pressures.. While;A a. filter may operate* satisfactorily to preventie capillary passage of liquidsl therethrough, it isf possible that t'he-pressur'e of the-liquid may be such ast@ mechanically force the liquid through the pores i'fll'ereoiv as il? the;` pores were orices; Itis' accordingly an. object of thetinvention to provide a filter that effectively blocks' liquidbut permits free fiowf of gases. therethrough.

Another.'V object is: toprovide a lter that' op'- pose's capillaryv passage. 01 liquidsI therethrough.Y

AnL additional object is to provide a. none-Wet- `table coating: on the. lterv surfaces that will bie durable: over: relatively long' periodssof time.; Ar further;l object is to' providef a non-Wettable coating' onV the pore surfaces of a lter: offsuch thickness that will not. close the pores.

Still` another objecty is' to provide a.- filter; for blocking liquids: at, ai predetermined maximum having' only gasgcommunicationwithithe, exterior IluztherA objects; and advantages of the inventioir will becomesa'i'llfarent:ass.thel description;` pito:- ceeds Whileith'e scope.T of the VVinvention ist pointed: out ciaims;. it'v may bestbc understood fromv a consideration' of' a speciic embodiment thereofY takeny in-v connection W-ith the accompanying drawing. in: Whichr Fig'urei-l-v shows afront elevation` of: a closed chamber embodying. the. invention;

Figure:V 2' shows' a:` vertical: cross section. along the-.lineiZ-z. of Figure 1- and depicts the mounted relationship of my" novel, ltereand. the?. closed chamber-with which it; is associated; andv z x Figure: 3 shows a greatly enlarg'ed representatiofnvofv'tvvc interecommuni'cating'poire spaces. that 'may be-includedfin anlterubody accordingtothe olifl pre's ent' inventionz Referring now to the drawing which depicts for illustrative purposes only one embodiment of my invention, there is shown in Figures 1 and 2 a chamber or receptacle having walls Il) in one of which is supported an insert I I. Suitable supporting means for the insert II comprise a cylinder I2 in threaded engagement with the walls of a passageway in one of the walls I of the chamber, and a threaded ring I3 engaging threads on the cylinder I2. The engagements between the insert II and the cylinder I2 and between this cylinder and a wall I0 of the chamber form air and watertight seals.

According to the invention the insert stitutes a porous body made of, for example, a ceramic which has been coated or impregnated in a novel manner to render it capable of transmitting air or gases therethrough but incapable of transmitting a fluid such as water.

Receptacles of this sort can serve a variety of purposes. For example, they might be used in Aassociation with humidors for tobacco, or as a casing for radio equipment. In connection with a humidor, the receptacle may serve as a cover therefor and be filled with Water with the insert I I disposed at the bottom thereof. No water will flow from the receptacle but water vapor will enter the humidor through the insert II and .provide desired moisture therein. As a casing for radio equipment, the receptacle is well suited for use under conditions where Water would nor- -mally wash over the equipment and where an airtight casing may be undesirable because of pressure requirements or for other reasons.

While attempts have been made heretofore to condition a porous body so as to make it waterproof in the sense that it excludes the entry of -water in its pores, many difliculties have been encountered. For example, one of the characteristics that the coating should possess is some deygree of permanence so that thecoating will persist over relatively long periods of time, say at least a year. Another feature that it should --have is one that prevents it from clogging the pores by closing the relatively narrow portions ythereof as by bridging opposite walls of such portions. In addition, the coating should Wet the walls of the pores while being itself unwettable by a liquid such as water. So far as I am aware, `no coating has been found prior to my invention that satisfies all of these variant requirements.

Thus, while parafne will harden on the pore walls and will be relatively durable as a coating, it is difficult to prevent a clogging of the pores when applying it as a coating. With the use of 'paradine as a coating the pores adjacent to the surfaces of the porousy body become clogged .relatively fast on the application of the coating so that the inner pores are provided with no coating at all. On the other hand, oils that do not 4harden as rapidly or at the same temperature as parafne, have too low a value of persistence to be useful for the purpose indicated. Such oils dry up or evaporate relatively quickly after application as a coating on a porous body with the result that frequent're-ccatings are necessary in order to preserve the water-repellent character of the body. Thus, both parafne'and loils that have a more lasting liquid phase under varying temperature conditions are impractical for use as a waterproong coating for porous bodies.

According to the invention a relatively thin `coating is provided on all surfaces of the porous `body insert I I, including the inner surfaces of its pores, which coating comprises a non-Wettable oil of the silicone type, known to the trade as Dow-Corning Type 200 fluid, which has a viscosity of 200 centistokes at 25 C. or other silicone oils of a similar nature. Figure 3 shows on a greatly enlarged scale a portion of a pore included in the body of insert II after it has been coated in accordance with the invention. A porous body such as a ceramic, includes solid p0rtions I6 shown in cross section, which dene a plurality of spaces or pores I1, I8, which comv municate with each other through constrictions II con- Y complete closure or clogging of the pores is unor passageways I9. On the walls of the pores I1, is a relatively thin coating 2i] of a silicone oil Areferred to, which has been applied according to the invention.

Silicone oil of the type referred to is well suited for use in the coating method of the invention, whereby it is thinly applied to prevent clogging of the pores. In addition, it has a high value'of persistence so that its coating characteristics lasts over an appreciably long period of time, say a year or longer. f f

In addition to providing a novel article of manufactureV in the form of a durably waterproofed porous body having a number of useful applications indicated above, the invention also =provides a novel method of applying the coating to the porous body.

In accordance with the method of the invention, a porous body having pores of a desired size is soaked in a solution containing from 2 to 4% silicone oil ofthe type described, and 9896% of a solvent which may-comprise either carbon tetrachloride'or one of the aromatic naphthas,

until all surfaces of the body, including the inner surfaces of the pores,'are wet with the solution. The body is then removedfrom the solution-and drained well and baked in an oven at a temperature of from 120 C. to 160 C. for a period of from 2 to 5 hours. After this treatment a coating remains on the pore surfaces which has vthe desirable properties mentioned herein.

While silicone oil of the type described is known .to possess waterproofing characteristics, it has not been suggested for use in an application where desirable. This is probably because of the fact that an application of silicone oil only, Without the solvent referred to, resultsin the clogging of the smaller pores of a porous body. However, employing the silicone oil in the proportion indicated in a solvent of the type mentioned results in the application of a uniform and relatively thin coating of oil to all pore surfaces, which .is incapable of clogging the pores. Thesolvent of course is driven off during the baking operation.

While the method of the invention is effective in connection with porous bodies other than ceramics, it is preferred to use porous ceramic bodies because such bodies are appreciablynnaffected by oxidation, acid vapors or other usual -corrosive Vagents that may be present when the method is being carried out. Where special-applications require porous metal bodies the method of invention can alwaysv be employed to advantage. Y

Aflfurther aspect of the invention concerns the dimensions of the pores of a porousbody.

In someV applications the magnitude of these dimensions Vmaybe important. Forv example, if water in the liquid phase is to contact directly the porous body, the pore sizeof the body should be such that the pressure of the liquid is insuffi- .cientto force the liquid mechanically Vthrough the pores of the body as if theyfwere orifices.v A per,-

son skilled in the art can readily determine what pore size is suitable for a given Water pressure condition.

It is apparent that the invention provides a novel gas liquid filter which effectively bars the flow of liquid therethrough While permitting the ow of gas through its pores.

Itis of course understood that the foregoing description of an embodiment of my invention is presented for illustrative purposes only and that the invention is not limited theretobut possesses a scope particularly pointed out in the appended claims.

What I claim is:

1. A chamber having walls delining a conned space, at least a portion of said walls being in contact with liquid at a pressure lower than a predetermined value, means in said portion of said Walls preventing passage of liquid therethrough at a pressure lower than said predetermined value, said means permitting gases to pass therethrough and comprising a body having pores therein of predetermined dimensions to block the free passage of liquid therethrough at said pressure, and an unwettable coating on the surfaces of'said pores for blocking capillary passage of said liquid therethrough, said coating comprising a silicone oil having a viscosity of 200 centistokes at a temperature of about C. for easy entrance into said pores and without'bridging spaced ones of said surfaces, said coating being baked in situ for drying on said surfaces for prolonged adherence thereon under conditions of use'.

y 2. A filterfpermitting the flow of gas therethrough but barring the flow of liquid, comprising a body having pores throughout its extent and a relatively thin coating on the walls of said pores of a silicone oil, having a viscosity of about 200 centistokes at a temperature of 25 C. said coating having substantially the same thickness in regions within said body remote from its outer surfaces, as at regions of said body close to said outer surfaces, whereby said pores remain open and said coating is durable for a relatively long period of time.

3. A lter comprising a porous medium having relatively minutely dimensioned spaces dened by the pores thereof and a durable, non-wettable and non-clogging coating on the walls of said pores comprising a dried silicone oil rmly ad- 6 herent to said walls, said silicone oil when liquid having a viscosity of 200 centistokes at a temperature of 25 C. said coating on each of said walls having a depth less than one half the distance separating opposite ones of said walls.

4. A lter having surfaces wettable by water and a coating composition wetting said surfaces, said coating composition being non-wettable by water, whereby said surfaces are rendered nonwettable by water when said coatinghas been applied thereto, said coating composition prior to application comprising a solution including from two to four percent of a silicone oil having a viscosity of about 200 centistokes at 25 C. and' 98-96 percent of one of carbon tetrachloride and the aromatic naphthas, and said coating composition subsequent to application comprising only said silicone oil in dried form for durability ofthe order of one year under conditions of use.

5. A filter rcomprising a body having poresr therein and a non-wettable coating on the walls of said pores of a dried silicone oil having a viscosity when in the liquid phase of about 200 centistokes at a temperature of 25 C., whereby said coating penetrates said pores without closing the same, said' silicone oil being baked in situ on the surfaces of said pores for drying, whereby said coating adheres to said pores for an extended period of the order of months under conditions of use.

LAWRENCE RUSSEL SHARDLOW.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS OTHER REFERENCES Ind. 8a Engineering Chemistry, vol. 39, No. 11, November 1947, pages 1366, 1369 and 1371. 

